UK emissions IVA. Carb problems 850DP. Holley (which) or ???

Hi all.

Getting very frustrated with the carb now having spent many hours working on it and also reading up on how to set up, how to rebuild, how to diagnose etc etc so hoping you guys can help out.

I'm having troubles getting current carb which was supplied with engine about 8 years ago (supposedly new Holley 850DP 0-4781-6) to run properly at idle. Fuel flow into engine from carb just doesn't seem to work properly but no previous knowledge of Holleys to gauge it against. We have tried rebuilding, checking every orifice, new gaskets, replaced power valves, secondary linkage disconnected, secondaries properly closed, even swapped round metering blocks. Next step is power valve blanking plugs and trying smaller main jets, just to try to eliminate them from the investigation. Sometimes runs ok but not great, sometimes just runs horribly. Idle adjusters have only very little impact on how it runs. Engine only running in garage, not on road. Fuel pressure consistent 6psi, fuel levels in float bowls good. We are wondering if we just got a dud carb? When blipping the throttle the fuel (from accelerator pump?) just seems to 'drop' onto the throttle bodies (I have heard of Holley DPs described as fuel toilets, is this what they mean?)

Engine is quite wild 396, 4.100 bore, 3.75 stroke, Yates C3, Ford SVO dry sump only block.

Block is 1993 so looking to get down to 4.5% CO2 at idle.

Firstly, any clues what might be wrong with the carb?

Secondly, Is there any hope of getting this combo down to 4.5% at idle?

Thirdly, if not what would be the best/simplest way to achieve it for the test and for future MOT tests? 600 4150 with vacuum secondaries, or 4160 Street Avenger, or 370cfm, or Edelbrock, or ??? Could swap to this for each MOT and go back to the 850DP (if we can get it running sensibly) or something else more sensible for the rest of the year. Therefore would prefer to use same inlet manifold etc.

Thanks very much for any help.

Randy V

Staff member
Lifetime Supporter
That's an awefully large carb for street use.
First thought that comes to mind is how much manifold vacuum you have? Wild cams with a lot of overlap are a real challenge to make work on the street and meeting emissions requirements can be almost an insurmountable challenge.

Lets hear more about this engine ...
Thanks Big-Foot

Vacuum is around 8 at idle and currently power valve is 6.5 which we know is too high, hence going to put blanking plug in for testing. Long term we will try 3.5 power valve but want to get idle/ part throttle sorted first.

Replacing with smaller (say 750 or even 600) is possible but don't want to spend on another carb yet until I am confident I have identified the carb which is most likely to manage to meet the emissions test, which I stated wrongly in previous post. Correct emissions required are CO <= 3.5% and HC(Hydrocarbons) <= 1200ppm and block is 1992, not 1993.
Seems like an awfully big carb for that size engine too. Is your emission testing only at idle? Holley carbs come with notoriously too large idle fuel jets. Unfortunately on most Holley carbs they're not removeable. Either get a Quick Fuel 650 mechanical or buy QF metering blocks with replaceable idle jets that can be used on a Holley carb. The idle mixture screws only give minimal adjustment. Changing the idle fuel jets makes a much bigger difference. A big duration camshaft may be difficult to meet emission standards though.